In cigarette packing, a variable-capacity cigarette store is interposed between a cigarette manufacturing machine and a packing machine to compensate for any difference between the number of cigarettes produced and the number of cigarettes packed.
One example of a FIFO (First In First Out) variable-capacity cigarette store is described in Patent EP-0738478-B1, U.S. Pat. No. 5,413,213-A1, Patent Application WO-9944446-A1, Patent Application WO-02085144-A1, and Patent Application WO-03026988-A1. A store of the type described in the above documents comprises an input station and an output station arranged in series along a cigarette feed path; an endless conveyor belt having a conveying branch and a return branch; and an adjusting device for adjusting the lengths of the conveying and return branches in complementary manner. The conveyor belt is driven by at least one electric motor to feed the cigarettes along the conveying branch, and engages a series of fixed guides for keeping the conveyor belt in the desired position. The conveying branch of the store receives a stream of cigarettes from an output conveyor of a manufacturing machine, connected to the input station of the store, and supplies a stream of cigarettes to an input conveyor of a packing machine, connected to the output station of the store.
In Patent Application WO-02085144-A1, the store conveyor belt is driven by an electric motor, connected to a return pulley of the conveying branch at the output station, to run the store conveyor belt at exactly the same travelling speed as the packing machine input conveyor; and the adjusting device of the store comprises its own electric motor, which is operated by a central control unit to adjust the lengths of the conveying and return branches in complementary manner. More specifically, the electric motor of the adjusting device is controlled by the central control unit as a function of an error signal proportional to the difference between the rotation speed of a return pulley of the conveying branch at the input station, and the rotation speed of a return pulley of the manufacturing machine output conveyor at the input station.
In Patent Application WO-03026988-A1, the store conveyor belt is driven by a first electric motor, connected to a return pulley of the conveying branch at the output station, to run the store conveyor belt at exactly the same travelling speed as the manufacturing machine output conveyor, and by a second electric motor, connected to a return pulley of the conveying branch at the output station, to run the store conveyor belt at exactly the same travelling speed as the packing machine input conveyor; the adjusting device of the store comprises a freely-running carriage; and, as opposed to being adjusted by a specific motor, the lengths of the conveying and return branches are adjusted in complementary manner by the first and second electric motor simply tensioning the store conveyor belt.
Though ensuring correct operation of the store, both the control systems described above fail to provide for optimum performance in terms of speed and accuracy.